This invention relates to brushless synchronous machines in general and more particularly to an improved protection arrangement for protecting a brushless synchronous machine against overvoltages at the field winding which is connected to a rotating excitation rectifier which is fed from an exciter machine, where the field winding can be short circuited by means of a solid state switching element in the event of overvoltages.
As a rule, the protection arrangement for protecting against overvoltages in a brushless synchronous machine consists of a solid state switching element, particularly a thyristor, which is shunted across the field winding and which can be closed automatically by a voltage monitoring device, which is equipped, in particular, with Zener diodes or other threshold components. A resistor is sometimes arranged in series with the switching element. The secondary winding of the exciter machine, the excitation rectifier, the switching element, the resistor, if applicable, and the voltage monitoring circuit are mounted on the rotor of the synchronous machine. Therefore, when the machine is operating they corotate with the rotor and the field winding. As soon as the output voltage of the excitation rectifier exceeds a given, permanently set value due to a disturbance, the excitation rectifier is short circuited on the output side by the protection arrangement. Since thereupon field current is no longer supplied to the field winding, the synchronous machine can no longer deliver torque even though the armature current still flows. The synchronous machine and, therefore, also the driven processing machine are shut down.
As already mentioned, the occurrence of overvoltages in the rotating part of the synchronous machine is detected automatically. Likewise, the initiation of the short circuit at the field winding is automatic. An indicating signal which indicates an overvoltage or the short circuit, is not available outside the rotating part of the synchronous machine. Such an indicating signal is desired at the outside, however, if switching actions are to be initiated because of the disturbance, for instance, if the armature current of the synchronous machine or the primary current of the exciter machine is to be switched off. Such an indicating signal, however, is also desirable if the disturbance is to be indicated visually or acoustically.